Well, it differs a little from state to state, but here's the general process.
1) Get a temporary certificate. This involves an examination process. In your case, you'll want to get a certificate for Secondary English or History. The name for these certificates varies from state to state, but you can find out what you need to do it by calling your local NEA affiliate.
2) Check job postings in your local school district(s). Generally, these postings go up first in June, and then again in August. Postings in August are a better chance because by August districts are desperate to fill all their slows with qualified teachers.
4) Within a limited period of time (generally 3-5 years), you need to get a masters degree in Education. Sadly, Education is one of the easiest masters degrees out there. Many teachers earn their masters at night over the course of 2-3 years.
While this process varies from state to state, almost every state offers a temporarily or emergency credential, with the caveat that they expect you to get a masters within a few years. Alternately, you can take a balls to the wall masters program and get your masters in education within about 15 months (many universities offer these programs).